1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus for a vehicle, and more particularly to a pressure and vehicle speed sensitive type hydraulic power steering apparatus which controls steering assist force in response to an external load (a road reaction force) correlated with fluid pressure of working fluid supplied to a power cylinder mechanism and vehicle speed.
2. Description of the Related Art
Relating to a hydraulic power steering apparatus of this kind, for example, there has been known a power steering apparatus disclosed in Japanese Patent Application Publication (JP-B) No. 6-79895. The power steering apparatus is provided with a pump for pressure feeding an oil, a hydraulic motor connected to a tire wheel via a linkage and having a piston and a cylinder, and a rotary valve apparatus for controlling flow of the oil to the hydraulic motor. An output passage of the pump is communicated with an inlet side of a vehicle speed sensing valve, and an outlet side of the speed sensing valve is communicated with a low speed oil inlet passage and a high speed oil inlet passage. Further, the rotary valve apparatus has an internal valve member integrally formed with a steering shaft and a valve sleeve rotatably receiving the internal valve member and integrally rotating with a pinion. The steering shaft and the pinion are connected to each other via a torsion rod displacing in a torsional manner, and first and second valve apparatuses are constituted by the internal valve member and the valve sleeve. In this case, the first valve apparatus increases the magnitude of pressure distributed to the hydraulic motor in correspondence to an increase of the torsional displacement, and the second valve apparatus controls a returning amount of the oil to a suction side of the pump in correspondence to the torsional displacement. Then, the speed sensing valve distributes the oil only to the first valve apparatus when the vehicle is at a low speed, and distributes the oil to both of the first and second valve apparatuses when the vehicle is at a high speed, whereby it is possible to obtain a proper assist force in correspondence to the speed of the vehicle and the torsional displacement.
Since there is no returning amount of the oil during low speed operation, the assist force is increased and operation of a steering handle becomes light. Since a predetermined amount of oil is returned to the suction side of the pump via the second valve apparatus during high speed operation, an amount of oil distributed to the hydraulic motor is reduced by the degree of the returning amount, the assist force is reduced, and the operation of the steering handle becomes heavy.
In this case, although the assist force (the steering assist force) with respect to the displacement is different, a characteristic of the assist force with respect to the torsional displacement is definitely determined in accordance with a geometrical shape of measuring edges of the first and second valve apparatuses, at every vehicle speed. However, since the characteristic of the assist force obtained by the same geometrical shape is not always optimum in all the vehicle speeds, it is desirable to increase freedom for setting the assist force in correspondence to the change of the vehicle speed in such a manner that the assist force can be set with every change of the torsional displacement at every vehicle speed.
The present invention relates to the matter mentioned above. An object of the present invention is to provide a hydraulic power steering apparatus for a vehicle which recognizes an external load, that is, a road reaction force by a fluid pressure of a working fluid supplied to a power cylinder mechanism, whereby freedom for setting a steering assist force in correspondence to vehicle speed is great.
In accordance with the present invention, there is provided a hydraulic power steering apparatus for a vehicle comprising,
an input member interlocking with a steering wheel, and
an output member connected to the input member so as to freely move relatively.
A steering mechanism steering a steered wheel is in an interlocking relationship with the output member.
A main control valve has a first valve portion and a second valve portion in which a valve opening degree is controlled on the basis of relative movement between the input member and the output member.
A power cylinder mechanism generating a steering assist force applied to the steering mechanism is in correspondence to a fluid pressure in a working chamber in which a working fluid is supplied and discharged.
A first flow passage communicates a pump apparatus for discharging a fixed flow amount of working fluid with the main control valve.
A second flow passage is branched from the first flow passage and communicates with a low pressure portion via an assist control valve and the main control valve.
A control unit controls the assist control valve.
The first valve portion controls a supply amount of the working fluid in the first flow passage to the working chamber, and
the second valve portion controls a discharge amount of the working fluid in the second flow passage to the low pressure portion in accordance with cooperation with the assist control valve.
A vehicle speed sensor for detecting a vehicle speed and a pressure sensor for detecting a fluid pressure of the working fluid in the first flow passage are provided. The control unit controls a valve opening degree of the assist control valve to a set valve opening degree set on the basis of the vehicle speed detected by the vehicle speed sensor and the fluid pressure detected by the pressure sensor.